Thermohaline Circulation: Thermocline, Halocline, Deep Water Formation and AMOC

Thermohaline Circulation: Meaning and What Drives It

Thermohaline Circulation: Meaning, Names and Two Limbs

Oceanographers describe thermohaline circulation as the part of the ocean's large-scale circulation that is driven by global density gradients, which form as the surface gains and loses heat and fresh water. The name joins thermo, for temperature, and haline, for salt content, the two factors that together fix the density of seawater. Winds drive the currents of the upper ocean, yet currents also flow thousands of metres below the surface, and those deep currents move because water of different density sinks and rises.

The same system has other names. Climate scientist Wallace Smith Broecker coined the term global conveyor belt, and it is also called the meridional overturning circulation, or MOC. Oceanographers use the second name because it is hard to separate the parts of the circulation driven by temperature and salinity from those pushed by wind and tides. It has two major limbs, the Atlantic Meridional Overturning Circulation (AMOC), centred in the North Atlantic, and the Southern Ocean overturning circulation near Antarctica.

Seawater Density: Why Cold and Salty Water Sinks

Density is what separates one layer of the ocean from another, and three properties set it.

• Temperature: Water becomes less dense as it warms, because the distance between its molecules increases.
• Salinity: Water becomes denser as salinity rises, because a larger mass of salts is dissolved in the same volume.
• Pressure: It matters little, because seawater is almost perfectly incompressible.

Fresh water is densest at 4 °C, but seawater only gets denser as it cools, right down to its freezing point, and that freezing point is lower than for fresh water, about −2 °C at typical salinity. So the coldest seawater is also the heaviest. Surface salinity rises when water is lost to evaporation or to ice and falls when fresh water arrives from rivers, rain or melting ice; a sea that cools and grows saltier at once becomes dense enough to sink, which is why deep water forms only in a few cold places.

There is a second reason the sinking happens near the poles and not at the equator. Experiments by Johan Sandström in 1908 showed that currents driven by heat require heating at a greater depth than cooling. The sun heats the ocean from above and makes the surface layer lighter, so tropical heating cannot set off sinking. Cooling at the surface in the polar seas can.

Surface Currents and Deep Currents Compared

Ocean currents are like rivers in the ocean, a regular volume of water moving along a definite path. The primary forces that start the movement are heating by solar energy, wind, gravity and the Coriolis force. Heating makes water expand, so near the equator the ocean stands about 8 cm higher than in the middle latitudes, and water flows down this slight slope. The Coriolis force turns moving water to the right in the northern hemisphere and to the left in the southern, producing large circular currents called gyres. Differences in water density affect the vertical movement of currents, and that vertical part is the thermohaline part.

Ocean Stratification: Thermocline, Halocline and Pycnocline

Mixed Layer and the Three-Layer Ocean

Temperature does not fall evenly with depth. In the low and middle latitudes the temperature structure of the ocean is a three-layer system, with warm water on top, a band where temperature drops sharply, and very cold water below.

• Top layer: Warm water about 500 m thick, at 20 to 25 °C. In the tropics it is present all year; in the middle latitudes it develops only in summer.
• Middle layer: A band 500 to 1,000 m thick in which temperature falls rapidly with depth.
• Deep layer: Very cold water extending to the ocean floor. About 90 per cent of the ocean's volume lies below the thermocline, at temperatures approaching 0 °C.

The top layer is the mixed layer, a layer in which active turbulence has homogenised a range of depths. Wind and waves stir it, and cooling or brine added from freezing sea ice makes surface water denser so that it convects downward. The mixed layer matters for climate because the top 2.5 m of the ocean holds as much heat as the entire atmosphere above it. The deepest mixed layers, exceeding 2,000 m, occur in regions such as the Labrador Sea, where convection, not wind, creates them.

The thickness of these layers varies with place and season. In the low and middle latitudes the warm top layer is commonly given as about 500 m thick and the thermocline as 500 to 1,000 m thick, although the thermocline can begin as shallow as 100 to 400 m below the surface. Oceanographic measurements put the mixed layer at a few metres to several hundred metres and the permanent thermocline from 200 to 300 m down to about 1,000 m.

Thermocline Layer: Meaning, Depth and Uses

A thermocline is the transition layer between the warmer mixed water at the surface and the cooler deep water below, and the temperature drops suddenly across it. In lakes the same layer is called the metalimnion. The temperature difference through the permanent thermocline can be as large as 20 °C, depending on latitude.

• By latitude: The thermocline is semi-permanent in the tropics, variable in temperate regions, and shallow to nonexistent in the polar regions, where the water column is cold from surface to bottom.
• By season: Its depth and strength change from season to season and year to year.
• Sound: The sudden change in density reflects sonar and other acoustic signals, which makes the thermocline important in submarine warfare.
• Cyclones: Forecasters look at the depth of warm water above the thermocline, not only at the sea surface temperature. Water vapour evaporated from the ocean is a hurricane's primary fuel, and the depth of the thermocline measures the size of the fuel tank.
• Lakes: In summer a lake has a warm layer on top, a cold layer below and a thermocline between them, and oxygen below the thermocline falls as the summer goes on because that water never reaches the surface.

Halocline: Meaning, Depth and Where It Forms

A halocline is a layer in a body of water where there is a sharp change in salinity with depth. Salinity at depth changes very little, because no evaporation, ice formation, rain or river water reaches it, while the surface grows saltier through evaporation and ice formation and fresher with rain and river water. Salinity generally increases with depth, and the zone where it increases sharply is the halocline. Other factors being constant, higher salinity makes seawater denser, so salinity alone can produce stratification.

• Where it forms: Most commonly where fresh water from rivers or melting ice mixes with salty ocean water, in estuaries, fjords and polar seas.
• Depth: In the Arctic Ocean the halocline lies between 50 and 250 m; in shallow seas such as the Baltic it is at 60 to 80 m.
• Role in the polar seas: There the surface water is colder than the deep water, so it is the halocline that keeps the water column stable and isolates surface from deep water. It also allows sea ice to form and limits the escape of carbon dioxide to the atmosphere.
• Mid-latitudes: Where evaporation exceeds precipitation the surface is saltier than the deep, so temperature supplies the stability and the salinity layer works against it.

Pycnocline Compared with Thermocline and Halocline

Density depends on both temperature and salinity, so the layer where density changes fastest has its own name. The pycnocline is the layer where the density gradient is greatest within a body of water. Below the mixed layer a stable density gradient separates the upper and lower water and hinders vertical transport.

In the low and middle latitudes a permanent pycnocline lies between 200 and 1,000 m and follows the thermocline. In subpolar and polar regions the surface waters are much colder and fresher, so there is no permanent thermocline, a permanent halocline exists, and that halocline is the main factor setting the pycnocline. Which property rules the surface depends on latitude: In the tropics and mid-latitudes surface density follows temperature, and at latitudes above 50° it follows salinity, because the water is already close to freezing.

Ocean Stratification and Why It Blocks Mixing

Ocean stratification is the natural separation of the ocean's water into horizontal layers by density. It is generally stable, because warm water floats on cold water and heating comes mostly from the sun. Wind-forced mixing reduces it. Stratified layers are a barrier to mixing, which limits the exchange of heat, carbon, oxygen and nutrients between the surface and the deep.

• Nutrients: The separation prevents nutrients from the lower layer reaching the upper layer, where phytoplankton grow.
• Seasons: In summer a seasonal pycnocline forms above the main one, and it is worn away again by surface heat loss and wind mixing before the next summer.
• Not a seal: Mixing across a pycnocline still occurs through turbulence, and it plays a key role in transporting nutrients.

Deep Water Formation in the Polar Seas

Ways Surface Water Becomes Dense Enough to Sink

Surface water sinks when it becomes denser than the water beneath, and only a few processes can do that on a large scale. Sunk water keeps its temperature and salinity for centuries, so each sinking region produces a water mass, a body of water with its own temperature and salinity signature that can be traced as it moves.

• Winter cooling and convection: In open-ocean convection, strong winter heat loss makes surface water dense and it sinks as a column; this occurs in the Labrador Sea and in the Greenland Sea.
• Brine rejection: When salty water freezes, the salt does not fit into the ice crystals and is expelled, so the water below the forming sea ice becomes saltier and denser. Sea ice has a salinity from 0 at its surface to 4 at its base on the practical salinity scale, and the brine drains downward through narrow channels in the ice.
• Evaporation in a restricted sea: In the Mediterranean, evaporation raises salinity and the dense water leaves through the Strait of Gibraltar as an undercurrent.
• Net evaporation over the open ocean: The North Atlantic is a rare place where evaporation outweighs precipitation, partly because it is so windy.

Deep water therefore forms in only a few places. In the north these are the Nordic and Labrador Seas; in the south, the Weddell and Ross Seas and other margins of Antarctica; and the Mediterranean is a side source. The Pacific is missing from the list. It receives large quantities of fresh rainfall, so its surface water is not saline enough to sink lower than several hundred metres, and its deep water must come from elsewhere.

North Atlantic Deep Water: Nordic Seas, Labrador Sea and Overflows

North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic. Warm surface water arriving from the south loses heat to the cold air, grows denser and sinks. The North Atlantic is salty because evaporation outweighs precipitation there, and moisture evaporated from the Atlantic is carried by the trade winds across Central America to fall as rain in the eastern North Pacific, while mountain ranges such as the Tibetan Plateau, Rocky Mountains and Andes prevent any equivalent moisture transport back.

NADW is a complex of water masses from two source regions:

• Labrador Sea: Labrador Sea Water, the upper part of NADW, forms by deep open-ocean convection in winter in the Labrador Sea and can reach depths of 2,000 m.
• Nordic Seas overflows: Two overflow waters, Denmark Strait Overflow Water and Iceland-Scotland Overflow Water, form the lower part of NADW; they spill over the Greenland-Iceland-Scotland Ridge from the Nordic Seas. In the Greenland Sea, one of the Nordic Seas, an ice tongue called the Odden once formed in winter, and the salt rejected from its ice made the water dense enough to sink to 2,500 m or more; since the 1990s it rarely develops.

NADW has a temperature of 2.0 to 3.5 °C, lies between 1,500 and 4,000 m and is detected by its high salinity and high oxygen content. It cannot flow towards the Pacific, because the Bering Strait is too narrow and shallow, so it flows slowly south into the deep abyssal plains of the Atlantic. Ridges, sills and straits steer these deep flows, so the shape of the ocean floor affects where deep currents can go.

How much of the Labrador Sea water joins the main overturning is debated. Increasing evidence suggests that water in the Labrador and Irminger Seas mainly recirculates within the North Atlantic gyre and has little connection with the rest of the AMOC, while NADW formation takes place primarily in the Nordic Seas.

Antarctic Bottom Water: Polynyas and Brine Rejection

The densest water in the open ocean forms around Antarctica. Antarctic Bottom Water (AABW) is formed in the Weddell and Ross Seas, off the Adélie Coast and by Cape Darnley. Cold winds blowing off the continent push newly formed sea ice away from the coast, opening stretches of open water called polynyas. The exposed water is cooled strongly by the winter air and freezes again, and each round of freezing makes the surface water saltier and denser by brine rejection. Coastal polynyas form as much as 10 per cent of the Southern Ocean's sea ice in a season.

• Temperature range: AABW ranges from −0.8 to 2 °C.
• Place in the water column: As the densest water mass it occupies the depths below 4,000 m in every ocean basin connected to the Southern Ocean, and it flows beneath NADW.
• Where it spreads: AABW formed in the Weddell Sea mainly fills the Atlantic and Indian basins; AABW from the Ross Sea flows towards the Pacific.
• Oxygen: Because it forms at the surface it is rich in oxygen, and its sinking ventilates the deep ocean.

Fresh meltwater is lighter than brine, so it resists sinking. Climate change and melting of the Southern ice sheet have slowed AABW formation, and the slowdown is likely to continue. Studies disagree on how strong this effect is, and some indicate that wind-driven sea ice formation overcompensates for melting of ice sheets in the Weddell Sea.

Antarctic Intermediate Water, Mediterranean Outflow and Water Masses Compared

Two water masses form at middle depths and show that sinking does not need extreme cold or extreme salt alone.

Antarctic Intermediate Water (AAIW) is a cold water mass of relatively low salinity. It forms at the Antarctic Polar Front zone, normally between 50° S and 60° S. Surface water moving north meets warmer subantarctic water at the front, and it sinks because it is denser than the water to its north and lighter than the water to its south. It is unusual because it is a sinking water mass with low salinity. Its salinity minimum is recognised throughout the Southern Ocean at 700 to 1,200 m, and its temperature on formation is 3 to 7 °C. It spreads north as far as 20° N, the largest spreading intermediate water.

The Mediterranean outflow is the opposite case, a warm and very salty water. Evaporation greatly exceeds precipitation and river runoff in the Mediterranean, and the average salinity of the basin is about 38‰ at 5 m depth. The dense water leaves through the Strait of Gibraltar as an undercurrent of about 1 sverdrup (Sv), a flow of 1 million cubic metres a second, while less saline Atlantic water flows in above it. In the Atlantic it sinks gradually to an equilibrium depth of about 1,100 m and spreads as the Mediterranean Salt Tongue, visible as a salinity anomaly at 1,000 to 1,200 m across the North Atlantic.

Deep water inside the Mediterranean forms in the Adriatic, the Aegean and the Gulf of Lion, driven by winter convection under cold winds such as the Bora and the Mistral.

Global Conveyor Belt: Path, Speed and Importance

Thermohaline Conveyor Belt Path Through the Four Oceans

The global conveyor belt links every ocean basin in one loop. Its path is easiest to follow as a sequence that begins where water first sinks.

1. Sinking: In the far North Atlantic, surface water is chilled by the cold air, and in places made saltier where sea ice forms, so it becomes dense and sinks towards the ocean bottom. Surface water moves in to replace it, which creates a current.
2. Southward flow: The deep water moves south between the continents, past the equator, down to the ends of Africa and South America.
3. Recharge: It travels round the edge of Antarctica, where the water cools and sinks again as it does in the North Atlantic, so the conveyor belt is recharged.
4. Branching: Two sections split off and turn north, one into the Indian Ocean and the other into the Pacific.
5. Upwelling: The branches warm up and become less dense as they travel towards the equator, and they rise to the surface.
6. Return: They loop back southward and westward to the South Atlantic and eventually return to the North Atlantic, where the cycle begins again.

Most of the deep water rises in the Southern Ocean, but the oldest water, with a transit time of about 1,000 years, rises in the North Pacific. Warm, fresher water from the tropical Pacific flows through the Indonesian archipelago, and the return flow of the upper cell passes mainly around the coast of Africa.

Upwelling, Speed and the Sverdrup

The conveyor is very slow and very large. It moves at a few centimetres a second, against tens to hundreds of centimetres a second for wind-driven or tidal currents, and any cubic metre of water takes about 1,000 years to complete the journey. It still moves more than 100 times the flow of the Amazon River.

Ocean transport is measured in sverdrups, and one sverdrup is about five times what the Amazon carries at its estuary. At the southern end of the Atlantic, around 80 per cent of the southward-moving NADW upwells in the Southern Ocean. After upwelling, water takes one of two paths: Water that surfaces close to Antarctica is cooled by sea ice and sinks back into the lower cell, while water that upwells in ice-free latitudes moves north and joins the upper cell.

Upwelling is slow and widespread. Its speeds are so low, even compared with the movement of bottom water, that it is hard to measure where it occurs from current speeds alone. Broecker's box models placed the bulk of deep upwelling in the North Pacific; computer models increasingly place most of it in the Southern Ocean, in the strong winds between South America and Antarctica.

Heat, Nutrients and Carbon: Why the Conveyor Matters

The conveyor matters in three ways.

• Heat for the poles: It supplies heat to the polar regions and so helps regulate the amount of sea ice there, although poleward heat transport outside the tropics is considerably larger in the atmosphere than in the ocean.
• Food chains: Warm surface waters are depleted of nutrients; they are enriched again as they travel through the conveyor as deep or bottom layers, and the base of the world's food chain depends on the cool, nutrient-rich water that returns to the surface.
• Carbon sink: Upwelled water is typically 1,000 years old and has not been exposed to the rise of carbon dioxide in the atmosphere, so it absorbs more carbon than surface water.

The conveyor has changed climate before. The Younger Dryas, a cold return between 12,800 and 11,700 years ago, came with an abrupt slowing of the AMOC caused by freshening from loss of ice on the Laurentide ice sheet. The explanation scientists historically favoured was a surge of meltwater from Lake Agassiz in North America. Later work finds that the floodwater followed the Mackenzie River, and what weakened the circulation first is still debated. The lesson holds either way: A burst of fresh water at the sinking sites can change climate within decades.

Ocean Currents, Water Masses, Fishing Grounds and Navigation

Ocean currents shape coastal climate. Warm currents raise temperatures in cold areas and cold currents lower them in warm areas.

• Cool west coasts: West coasts of the continents in tropical and subtropical latitudes are bordered by cool waters; they have low temperatures with narrow ranges, fog, and generally arid conditions.
• Mild west coasts higher up: West coasts in the middle and higher latitudes are bordered by warm waters, giving a marine climate with cool summers and mild winters.
• Warm east coasts: Warm currents flow parallel to the east coasts of the continents in tropical and subtropical latitudes and give warm, rainy climates.
• Fishing grounds: The mixing of warm and cold currents replenishes oxygen and favours plankton, the primary food of fish, so the best fishing grounds lie mainly in these mixing zones.
• Coastal upwelling: Upwelling brings deep, cold, nutrient-rich water to the surface; the five major coastal upwelling currents are the Canary, Benguela, California, Humboldt and Somali currents, and about 25 per cent of the world's marine fish catch comes from five upwellings that cover 5 per cent of the ocean area.
• Navigation: Ships plan routes and dock with the help of current forecasts, because a current can speed a voyage or push a ship off course; its speed, called drift, is measured in knots and can exceed five knots at the surface.
• Rescue and oil spills: Rescue teams and spill responders use current patterns to predict where a missing person, a floating object or an oil slick will drift.

Currents and water masses work on different scales. Currents move water sideways near the surface and set coastal climate and fishing grounds over seasons and years. Water masses such as NADW and AABW sink, carry oxygen and heat into the deep ocean for centuries, and return nutrients to the surface when they finally upwell.

AMOC: Structure, Observations and Collapse Risk

AMOC Full Form, Limbs and Heat Transport

AMOC stands for Atlantic Meridional Overturning Circulation. It is the main ocean current system of the Atlantic and half of the global thermohaline circulation, the other half being the Southern Ocean overturning circulation. Meridional means along the lines of longitude, north to south, and overturning means that water sinks in one place and rises in another.

• Upper limb: Warm, more saline water flows north in the upper layers. It is salty because evaporation is high in the tropics.
• Overturning: When this water cools in the north its density rises and it sinks.
• Lower limb: Cold, less salty deep water returns south.
• Links: The two limbs are linked by regions of overturning in the Nordic Seas and the Southern Ocean.

The Atlantic is the only ocean in which heat flow is northward. Much of the transfer happens through the Gulf Stream, a surface current that carries warm water north from the Caribbean. The Gulf Stream as a whole is driven by winds alone, but its northernmost segment, the North Atlantic Current, obtains much of its heat from the thermohaline exchange in the AMOC. The AMOC carries up to 25 per cent of the total heat towards the northern hemisphere.

A popular claim holds that without the AMOC northern Europe would be as cold as northern North America, 15 to 20 °C colder. That idea is generally considered incorrect, because atmospheric patterns also carry heat. The consensus is that the AMOC keeps northern and western Europe warmer than it would otherwise be by between 4 °C and 10 °C, depending on the area.

Direct Observation of the AMOC at 26° N

The AMOC is measured directly by a line of moored instruments called the RAPID array, which has made continuous measurements at 26° N in the Atlantic since April 2004. It combines current meters, subsea cable measurements of the Florida Current and estimates of the flow in the ocean interior from temperature and salinity measurements, to give continuous, full-depth, basin-wide estimates. Its total is the sum of three parts.

• Florida Current: The Gulf Stream transport through the Florida Strait, between 30.4 and 32.4 Sv in the annual means.
• Ekman transport: The northward surface transport driven by wind stress.
• Mid-ocean transport: The flow between the Bahamas and the Canary Islands, which runs southward and is entered as a negative number in the annual means.

The record surprised oceanographers. The AMOC had been thought to vary slowly on climate timescales, but RAPID showed large variability on timescales of days to a decade, driven largely by wind forcing. Annual means fell from 20.9 Sv in 2005‑06 to a low of 13.5 Sv in 2009‑10 and were 17.8 Sv in 2017‑18. The record to 2012 appeared to show a decline about 10 times greater than the most advanced models of the time predicted, which started a debate on whether it showed climate change or large natural variability. An analysis of the 26° N record published in 2020 found that the AMOC is no longer weakening, though the recent transport is not above the long-term mean.

Two cautions apply. A record of about two decades is short against a circulation whose timescale is centuries, and the observations come largely from a warm phase of the Atlantic multidecadal variability. Reconstructions from before 2004 disagree as well: Some find a weakening of around 15 per cent since the mid-20th century, others find no change, and a 2022 review gave the size of change since 1980 as uncertain, between 5 per cent and 25 per cent.

AMOC Slowdown: What the IPCC Assessed

Warming and melting both work against the AMOC. Climate change makes surface water warmer and, with melting of the Greenland ice sheet and increased precipitation over the North Atlantic, less saline. Both changes widen the difference between surface and deep layers, which makes the sinking and the rising that drive the circulation more difficult. Warm, fresh water sinks less readily than cold, salty water.

The IPCC's Sixth Assessment Report states the position in graded language, and the confidence words are part of the answer.

• Very likely to weaken: The AMOC is very likely to weaken over the 21st century for all emissions scenarios, with high confidence in the decline but only low confidence in its size.
• Medium confidence on collapse: There is medium confidence that there will not be an abrupt collapse before 2100.
• If it did collapse: It would very likely cause abrupt shifts in regional weather patterns and the water cycle, such as a southward shift in the tropical rain belt, weakening of the African and Asian monsoons and strengthening of Southern Hemisphere monsoons, and drying in Europe.

The Fifth Assessment Report of 2014 had judged a rapid transition very unlikely with high confidence; the Sixth is less certain about avoiding collapse, a change likely influenced by studies pointing to a stability bias in general circulation models.

AMOC Collapse: Tipping Point Debate

An AMOC collapse would not be reversible on human timescales, and so it counts as one of the tipping points in the climate system. The idea comes from Henry Stommel's 1960s box model, which showed that the AMOC could exist in a strong state like the one throughout recorded history or collapse to a much weaker state and not recover unless the warming or freshening that caused the collapse is reduced.

The evidence points in different directions.

• Comprehensive models: The general circulation models usually show a single stable state and a circulation that weakens but recovers. Researchers have raised concern that this stability may be partly a model bias.
• Simpler models: Intermediate-complexity models can show collapse. A 2023 statistical analysis of such models suggested the most likely collapse around 2065, with 95 per cent confidence between 2037 and 2109; it drew heavy criticism, because the models are considered less reliable and its proxy record, from the Northern Subpolar Gyre, was called insufficient.
• Recent multi-model work: A study published in Nature in February 2025 concluded that the AMOC is resilient to extreme greenhouse gas forcing and to large inflows of fresh water into the North Atlantic across 34 climate models, suggesting that collapse is unlikely in the 21st century.
• Tipping assessment: A 2022 assessment of climate tipping points found that collapse would most likely be triggered by 4 °C of global warming, with uncertainty from 1.4 °C to 8 °C, and that once triggered it would take between 15 and 300 years, most likely around 50.

Climate Effects of a Weaker Thermohaline Circulation

Europe and the North Atlantic

A shutdown of the AMOC is expected to cause substantial cooling in Europe, particularly in Britain and Ireland, France and the Nordic countries. The 2022 tipping assessment concluded that a collapse would lower global temperature by around 0.5 °C, while regional temperatures in Europe would fall by between 4 °C and 10 °C. A partial slowing would cause less, and some scientists believe it would give limited cooling of around 1 °C in Europe. These figures are modelled ranges and sit on top of continued warming, so the net temperature change would differ from place to place.

Past events show the mechanism. During the Younger Dryas, Northern Hemisphere temperatures returned to near-glacial levels, possibly within a decade, while the rain belt shifted south and Europe dried. A weaker AMOC reduces average air temperatures over Scandinavia, Great Britain and Ireland, because these regions are warmed by the North Atlantic Current.

Monsoon, Rainfall and the Tropical Rain Belt

The AMOC is not a distant Atlantic matter for India. A weakened AMOC would displace the Intertropical Convergence Zone southward, as happened during the Younger Dryas, and the IPCC lists a southward shift of the tropical rain belt, weaker African and Asian monsoons and stronger Southern Hemisphere monsoons among the effects of a collapse.

If the planet continues to warm, fresh water from melting polar ice could shift the tropical rain belt and cause droughts for millions of people. Two cautions apply. These monsoon effects are conditional on a collapse, which the IPCC does not expect before 2100 with medium confidence, and the size of the effect on Indian rainfall is not quantified in the assessment.

Sea Level, Ocean Heat and Oxygen

Weakening of the AMOC would accelerate sea level rise along the United States East Coast. A weaker AMOC carries less heat on towards Europe, so the waters off the US East Coast warm and expand; this is one of the reasons sea level rise along that coast is estimated to be three to four times the global average.

Layering of the upper ocean is also rising. Between 1960 and 2018 upper ocean stratification increased by 0.7 to 1.2 per cent per decade. The Southern Ocean showed the strongest rise, followed by the Pacific, Atlantic and Indian oceans, and temperature drives most of the change, with salinity important only locally.

• Oxygen loss: There is high confidence that oxygen levels have dropped in many upper-ocean regions since the mid-20th century.
• Nutrient supply: Stronger layering limits the supply of nutrients to the sunlit surface; a 2005 paper found that severe AMOC disruption could cut North Atlantic plankton counts to less than half of normal biomass.
• Outlook: The IPCC finds it virtually certain that upper ocean stratification will continue to increase in the 21st century.

Indian Ocean Circulation and the Conveyor

Indian Ocean: No Deep Water of Its Own and Monsoon-Reversed Currents

The Indian Ocean makes no deep water of its own. Asia blocks heat export and prevents ventilation of its thermocline, so its deep water arrives from the south. The inflow of deep water into the Indian Ocean is about 11 Sv, most of it Circumpolar Deep Water. North Atlantic Deep Water reaches into the Indian Ocean south of Africa at a depth of 2,000 to 3,000 m, and Antarctic Bottom Water, deeper still, moves north through the Crozet-Kerguelen Gap at 2.5 Sv, 23 years after it forms.

• Monsoon currents: The Indian Ocean monsoon, the strongest on Earth, reverses the Somali Current and the Indian Monsoon Current with the seasons; in the winter monsoon, from November to February, circulation is reversed north of 30° S.
• Arabian Sea upwelling: Cold water rises near the Horn of Africa and the Arabian Peninsula in the northern hemisphere.
• Gyres: The dominant flow pattern is a clockwise gyre north of the equator and an anticlockwise gyre south of it, including the Agulhas Current.

Indonesian Throughflow, Return Flow and Indian Ocean Salinity

The Indonesian Throughflow (ITF) is the low-latitude movement of warm, relatively fresh water from the tropical Pacific to the Indian Ocean, and it serves as a main upper branch of the global heat and salt conveyor belt. The sea surface of the far western Pacific stands higher on average than the adjacent Indian Ocean, and the difference drives water downhill through the Makassar Strait and the Banda Sea, and out through the Lombok, Ombai and Timor passages. Total outflow corresponds to 15.0 Sv, varying from 10.7 to 18.7 Sv.

The reason it matters for the conveyor is balance. The cold, salty water that sinks and flows out of the Atlantic is replaced by warmer, fresher upper-ocean water that travels from the tropical Pacific through the Indonesian archipelago, then across the Indian Ocean and round Africa, up the Atlantic towards Greenland. There it cools, sinks and starts the circuit again.

The average salinity of the Indian Ocean is 35‰. The Bay of Bengal records lower salinity because of the influx of river water, and the Arabian Sea shows higher salinity because of high evaporation and low influx of fresh water. Surface salinity is higher than 36‰ in the Arabian Sea and about 33‰ in the Bay of Bengal. Peninsular India separates the two basins.

Southern Ocean Overturning and Ocean Monitoring

Southern Ocean Overturning Circulation and Argo Floats

The Southern Ocean overturning circulation is the southern half of the global thermohaline circulation. It has two cells. The smaller upper cell is most strongly affected by winds because it is near the surface, while the larger lower cell is defined by the temperature and salinity of Antarctic Bottom Water. The strength of both cells has changed: The flow of the upper cell has increased by 50 to 60 per cent since the 1970s, while the lower cell has weakened by 10 to 20 per cent. Part of this change comes from the natural Interdecadal Pacific Oscillation, and climate change has played a substantial role, because fresh meltwater from the Antarctic ice sheets dilutes the salty bottom water.

When the formation of dense, cold water weakens near the coast and the flow of warm water towards the coast strengthens, surface water is less likely to sink and mix with lower layers, and stratification rises. One study suggests that the circulation would lose half its strength by 2050 under the worst climate change scenario. Researchers also consider that the less-studied Southern Ocean circulation may be more vulnerable to collapse than the AMOC, although the certainty is far lower than for most other tipping points.

The deep circulation cannot be seen from space, so it is observed with instruments in the water. Argo is an international programme whose fleet is almost 4,000 drifting floats. Each float drifts at a parking depth of 1,000 m and, every 10 days, dives to 2,000 m and then rises to the surface, measuring conductivity, temperature and pressure. Scientists calculate salinity and density from these measurements, and seawater density is important for working out large-scale motions in the ocean. The data go to shore by satellite and are free to everyone.

Sources

• NCERT: Fundamentals of Physical Geography (Class XI), Water (Oceans)
• NCERT: Fundamentals of Physical Geography (Class XI), Movements of Ocean Water
• NCERT: Fundamentals of Physical Geography (Class XI), Solar Radiation, Heat Balance and Temperature
• NOAA National Ocean Service: What is the global ocean conveyor belt?
• NOAA National Ocean Service: Thermohaline Circulation
• NOAA National Ocean Service: The Global Conveyor Belt
• NOAA National Ocean Service: Effects of Climate Change
• NOAA National Ocean Service: What is a thermocline?
• NOAA National Ocean Service: What is the Atlantic Meridional Overturning Circulation (AMOC)?
• NOAA National Ocean Service: How Currents Affect Our Lives
• NOAA Repository: Moat and others (2020), Pending recovery in the AMOC at 26N
• IPCC AR6 Working Group I: Summary for Policymakers
• Wikipedia: Thermohaline circulation
• Wikipedia: Atlantic meridional overturning circulation
• Wikipedia: North Atlantic Deep Water
• Wikipedia: Antarctic bottom water
• Wikipedia: Antarctic Intermediate Water
• Wikipedia: Mediterranean outflow
• Wikipedia: Mediterranean Sea
• Wikipedia: Brine rejection
• Wikipedia: Halocline
• Wikipedia: Pycnocline
• Wikipedia: Thermocline
• Wikipedia: Ocean stratification
• Wikipedia: Mixed layer
• Wikipedia: Labrador Sea
• Wikipedia: Greenland Sea
• Wikipedia: Gulf Stream
• Wikipedia: Younger Dryas
• Wikipedia: Indian Ocean
• Wikipedia: Indonesian Throughflow
• Wikipedia: Southern Ocean overturning circulation
• Wikipedia: Upwelling
• Wikipedia: Sverdrup
• Wikipedia: Argo (oceanography)
• UPSC: Civil Services Examination question papers